Fossil Focus: Placodonts

Introduction:

The placodonts were a group of marine reptiles that lived in shallow coastal waters, and mostly ate hard-shelled prey such as mussels and other bivalves (that is, they were durophagous). They lived during the Triassic period, and have so far been found in modern-day Europe, the Mediterranean and South China (Fig. 1). The Triassic was a very special time for marine-reptile evolution, with their greatest morphological diversity being known from this period. The beginning of the Triassic was characterized by the largest mass-extinction event that has ever occurred on Earth (the Permian–Triassic extinction), in which around 95% of all marine life went extinct. This marked the start of the Mesozoic era (the ‘age of dinosaurs’ that contains the Triassic, Jurassic and Cretaceous periods, and dates from about 252 million to 66 million years ago). Ecosystems that had been dominated by amphibians became dominated by reptiles, many of which — including the ichthyosaurs, thalattosaurs, protorosaurs, saurosphargids and first turtles — evolved to become secondarily aquatic during the Early and Middle Triassic. However, the largest and most successful group was the sauropterygians, a clade that contained the placodonts, pachypleurosaurs, nothosaurs and pistosaurs in the Triassic, and the famous plesiosaurs in the Jurassic and Cretaceous. The placodonts are thus an important group to study, because they represent ‘primitive’ members of the Sauropterygia that are closer to their terrestrial ancestors than the more derived sauropterygians.

Anatomy:

Unlike more derived sauropterygians, placodonts did not have fins or particularly streamlined bodies; they retained relatively unmodified limbs with fingers and toes, similar to those of terrestrial reptiles. They had box-like bodies with a dense array of gastral ribs protecting their undersides, and were usually only about 1–2 metres in length. The placodonts can be divided into two morphotypes: the unarmoured ‘placodontoids’, which resembled reptilian sea cows and lacked a carapace but often had a series of osteoderms (bony deposits in the skin) on their torso (in Placodus and Pararcus); and the heavily armoured, superficially turtle-like cyamodontoids, which are more commonly found in the fossil record (Figs 1 and 2). Cyamodontoids either had one large carapace (as in Henodus and Glyphoderma) or two, with a large carapace protecting the majority of the trunk and a separate, smaller one that protected the pelvic region (as in Cyamodus hildegardis and Psephoderma; Fig. 1C and D). Apart from their armour, the most distinguishing anatomical features of the placodonts are their robust skulls and highly modified teeth, which were adapted for crushing hard-shelled prey such as bivalves and crustaceans. These teeth are extremely enlarged and flat, like slabs of stone, being characteristically angular in ‘placodontoid’ taxa and rounded, or bean-shaped, in cyamodontoids. They also have the unusual feature of being located on the enlarged palatine bone (that is, on the palate of the mouth), so the animal could crush shells incredibly efficiently (Fig. 3). A recent study has also shown that tooth replacement (which occurred throughout life; Fig. 3B) took place in a unique pattern in cyamodontoid placodonts, occurring unilaterally (on one side of the mouth at a time) and in functional crushing groups. This allowed placodonts to maintain the ability to crush prey items on at least one side of the mouth at all times, a method that has not previously been observed in any other reptile group.

Figure 2 — Phylogenetic relationships of placodont genera. All taxa shown are European, except Placodus, because no phylogenetic studies have yet been published on Chinese cyamodontoid taxa. This work is currently being conducted.

All placodonts appear to be specialized for a durophagous diet, with the exception of the very unusual Henodus from the Carnian (Late Triassic) of southern Germany. This animal lived in fresh water and/or brackish lagoons, and had an extremely reduced dentition, having only two small, rounded teeth in the upper jaw (palatine) and two in the lower jaw. The premaxillary teeth (those at the top and front of the mouth) were modified into a row of denticles that form a ‘beak’, and there is evidence that the jaws may have also supported baleen-like structures similar to the filtering structures of some whales, suggesting that this unusual animal was a filter feeder.

Figure 3 — CT reconstructions of four placodont skulls seen from the palate, showing the highly specialized crushing teeth of the upper jaw. Not to scale. Teeth are colour-coded according to their position: purple, palatine teeth; blue, maxillary teeth; green, premaxillary teeth; red, exposed replacement teeth visible because functional teeth are missing. A, the unarmoured ‘placodontoid’ Placodus gigas. B, Cyamodus kuhnschnyderi. Four teeth are missing in this specimen, three revealing replacement teeth underneath. C, Macroplacus raeticus. This taxon had by far the largest teeth of any placodont. D, Psephoderma alpinum, which had an elongated nose.

Palaeoecology:

It has been shown that placodonts evolved from a non-durophagous ancestor called Palatodonta which, like placodonts, had a row of teeth on the palatine (Figs 1A and 2). However, these teeth were pointed and sharp, much more suited to a diet of soft prey than of immobile, hard-shelled prey. This shows that the unusual condition of a palatine tooth row was not originally an adaptation for durophagy, but was only later adapted for such a diet by the placodonts.

The placodonts were the first group to branch from the rest of the Sauropterygia. Despite this, recent work has shown that they were comfortable living and feeding in the water, and that they were well adapted for such a lifestyle. Computed tomography (CT) scanning of two skulls of the non-armoured Placodus revealed the structure of the braincase and endocranium, including the inner ear (labyrinth organ), which is responsible for the sense of balance (Fig. 4). Much like the inner ear of modern marine reptiles, the semi-circular canals are very compressed and elongate, indicating an equally aquatic lifestyle for the placodonts. In addition to this, the position of the horizontal canals indicates Placodus was at its most alert and comfortable with its head pointing downwards at an angle of just over 20°, ideal for feeding on the sea floor.

Figure 4 — Reconstructions of the braincase and endocranium of Placodus gigas. A, rendering of a skull (left) with a transparent version (right) exposing the braincase elements. B, Virtual model of the endocranium (blue), inner ear (pink) and cranial nerves (yellow) in right lateral (top) and dorsal (bottom) views. C, Virtual model of the inner ear in right lateral (top left), caudal (top right) and dorsal (bottom) views. Note the compressed and elongate semicircular canals.

Placodont armour would not only have functioned as effective protection from predators, but would also have acted as ballast to weigh the animal down on the sea floor so it could feed with minimal effort. Some of the more derived placodonts, such as Psephoderma and Psephochelys, had toothless, elongate ‘beaks’ that would have been used to probe the sandy sea floor to root out hidden food items.

Fossil Record:

Placodonts are generally quite uncommon in the fossil record, with no reliable examples known from the Early Triassic and relatively few from the Late Triassic. Articulated remains are only known from localities of exceptional preservation (Lagerstätten) such as Monte San Giorgio on the border of Switzerland and Italy, or Guanling in southern China (Fig. 5). Outside of Lagerstätten, the most common remains are shed teeth, lower-jaw fragments, skull material and armour plates. Disarticulated limb bones, vertebrae, ribs and so on are also found, but it can be very difficult to determine which species these belong to without associated cranial material.

Summary:

The placodonts represent a unique and highly specialized group of marine reptiles. From an ancestor that had a double row of pointed teeth in the upper jaw (Palatodonta), the placodonts evolved a dentition that was incredibly well suited to an extreme durophagous diet, even developing a specialized method of tooth replacement that allowed them to feed constantly. Indeed, placodonts were well adapted to an aquatic lifestyle, despite branching early in sauropterygian history, and many taxa were well protected from predators by their heavy armour, which also aided feeding at the sea floor. Our understanding of placodont anatomy and palaeoecology has advanced in recent years, especially in light of discoveries in China, but much remains to be discovered.